Reverb Tank Hum Cancelling Coil

[Jon Fields]

I have a Mojotone Princeton Reverb where the reverb tank is picking up 60 Hz from the power transformer (not too obnoxious at reasonable reverb settings). Moving the tank out of the chassis eliminates the hum. Experimenting with placing the tank nearer the power transformer seems to indicate the open and closed side of the tank towards the amp chassis generate similar hum. Experimenting with various steel objects from my kitchen as shields does not seem to help. I know steel is not a top of the line mu-metal, but the tank chassis is also steel. Mu-metal is more expensive than reverb tanks!

I am considering buying a second reverb tank and scavenging it for the receive side pickup. Then I could consider mounting it in the same orientation near the main reverb tank's receiver. I would then wire it out of phase. If this was such a good idea, I wonder why no one makes one...

Thoughts?

Thanks,
Jon

 

[JohnRoberts]

Have you considered mu metal shielding?

JR

 

[Jon Fields]

Have you considered mu metal shielding?

JR

It may seem like dramatic overkill, but I could mount two tanks in the amp, one with only its output connected out of phase to act as a hum cancelling coil, for a lot less than mu metal to shield one tank (mu metal $ >> extra reverb tank $)

 

[JohnRoberts]

It could be noisy. You don't need a full second tank just to capture hum to subtract, just the second pickup. This doesn't sound cheap or easy, but do whatever floats your boat.

JR

 

[gyraf]

mu metal is only a factor 4-5 better than soft iron, often better to just add iron if you have enough space

shape is important, as what it does is to guide/conduct magnetic fields around and away from your sensitive spots

have you tried turning/moving the power transformer around in stead?

 

[Perm0]

Using dual tanks wired out of phase should also reduce the "poing" sound.
There used to be a website explaining multiple tank arrays and their benefits, but I can no longer find it.

 

[JohnRoberts]

+1 to steel/soft iron shielding .

When I was over mixer engineering for Peavey last century, we had lots of powered top box mixer with reverb tanks inside pretty tight packages. Tweaking the power transformer orientation and the tanks for best hum noise result was routinely done. Be aware that tanks are designed to be used in specific orientation wrt spring sag so be careful springs don't bump anything that they shouldn't if used incorrectly.

Later when the springs were replaced by digital efx boards, that simplified the hum pickup aspect of the top box designs.

JR

 

[user 133392]

Using dual tanks wired out of phase should also reduce the "poing" sound.
There used to be a website explaining multiple tank arrays and their benefits, but I can no longer find it.

That might have been here: https://www.proaudiodesignforum.com/forum/php/viewtopic.php?t=213

 

[Perm0]

Ah that's great .. Thanks !

I need to pick up on this, it was my plan some time ago to do this multiple tank setup, I build a stereo driver that can drive low impedance tanks, it has channel link for mono in stereo out and two high Z pickup amps with a bit of a tone control in the mid band. I drive it from a compressor and high pass EQ.

Got two tanks for stereo... and somehow left it there.

Sounds great, but it can be much better...

I need to pick up on this.

 

[vintageampfixer]

It may seem like dramatic overkill, but I could mount two tanks in the amp, one with only its output connected out of phase to act as a hum cancelling coil, for a lot less than mu metal to shield one tank (mu metal $ >> extra reverb tank $)

I love the thinking but doubt a second reverb would be sufficiently phase aligned with the first to allow its signal to be subtracted. I'd just move whatever's inducing the hum!

 

[CJ]

Those tanks are directional, you want the pickup transducer on the opposite side of the pwr xfmr, but I bet you figured that part out already.

You have about 2000 turns of sure on a high perm nickel core so it makes a good him antenna.

Certain amps with reverb are less noisy than others, Old Fender amps have always been a bit noisy but you don't notice once the guitar starts to dominate the signal to noise ratio.

 

[capacitorless]

If you have a 2nd tank (and assuming it's cheap enough) could the 2nd tank's pickup be removed, and positioned/wired next to the 1st like a hb guitar pickup, but without having a spring running through it?

I don't know how much hum/noise is picked up by the spring itself, though.

 

[Tubetec]

To make the inductive components as close as possible to each other in space with two tanks you need to rearrange the way the coils and cores are mounted .
To do it right you'd probably use one off the shelf tank , then get the manufacturer to custom make its mirror image , there is at least one tank manufacturer that allows you create custom configurations ,

Its possible to dismantle the spring and rearrange everything yourself , but it would be easier to have the factory assemble it the way you want .

Ive done a few experiments with taking a balanced hi-z output from the tank pickup coil , in conjunction with extra screening of the tank it vastly reduces susceptabillity to nearby SCR dimmer hash ,

 

[Perm0]

I noticed my tanks had all the possible mounting holes so it's really easy to swap direction of the insides and in and output connectors.
I've sandwiched two tanks so they form a sealed metal box with two springs inside for L/R

 

[Jon Fields]

Thanks for all the responses. I assume this closes the case on there ever being a reverb tank with a hum canceling coil on the recovery side. It either sounded bad or was more cheaply mitigated by moving the spring. When the Fender Bandmaster Reverb Head first came out, it was oversized to my eye likely for this exact problem. I imagine if there ever was a motivator for a commercial humbucking reverb spring, that would have been the one.

Since reverb settings I use have only enough hum that I know the amp is powered on in a quiet room with no guitar playing going on, I'm gonna resist gilding the lily and leave it be. Perfection is the enemy of good...

 

[CJ]

One thing you might try is making sure the cables are making good contact, I have fixed excessive reverb hum by hard wiring the cables to the tank and putting new RCA ends on the amp side of the fables, for some reason thus is a high corrosion area, maybe dissimilar metals are to blame.

 

[Tubetec]

The reverb signal from two tanks is not correlated , like a normal balanced signal would be , but the wanted signals still add and the noise still cancels .
The effect of combining the two tanks tends to iron out the flutter echos and smoothens the decay , it also creates a very good sheild when two tanks are enclosed .

A close look at the original fender reverb shows very good sheilding of the transducers from the mains transformer , amp heads are generally worse due to close proximity of tank and transformers , combos can do a little better .

 

[David Kulka]

Maybe remote the power transformer.

 

[Tubetec]

For studio use you can remote the tank on the speaker output of the guitar amp via an attenuator , then orientate the tank for minimum pickup of local noise sources , preferably with a balanced recovery circuit .